The team designed a developmentally regulated genomic targeting of DNA methylation specifically silencing the p16 promoter, a tumor suppressor gene that controls cell division, and whose inactivation is among the most common and earliest epigenetic events in human cancer.
Using an in vivo mouse model, the scientists could observe that mice carrying the methylation-inducing transgene construct had increased transcriptional suppression in somatic tissues during aging and also a significant increase in spontaneous cancers, such as lung cancer, leukemia, or sarcomas, while wild-type controls did not develop tumors. Furthermore, mice that were born with the methylation-inducing transgene also exhibited p16 promoter methylation and gene silencing, developing accelerated tumor onset and having considerably shortened tumor-free survival.
“This is the first demonstration that gene promoter methylation can actually cause cancer,” said Dr. Lanlan Shen, lead author of the study, as quoted in an article on The Scientist by Ashley P. Taylor.
Over the last 25 years there has been growing evidence associating epigenetic changes and cancer, since the majority of cancers present abnormal methylation patterns. Although specific mutations that add methyl groups to DNA, such as methylated cytosine residues in the p16 promoter, have been associated with certain tumor types, the fundamental role of methylation in carcinogenesis is still unclear.
“For many years we’ve been very convinced that DNA methylation changes and epigenetic silencing contribute to human cancer, and there have been a lot of observations that support that concept,” said Peter Jones, research director and head of the cancer epigenomics lab at Michigan’s Van Andel Research Institute. “What this paper does, which I think is very clever, is to selectively silence a tumor-suppressor gene in a mouse model system and then show that those mice do develop cancers. This shows that epigenetic silencing can lead directly to the formation of cancer.”
“It certainly shows that the methylation of the gene—and losing the gene this way—is important to something that happens that might be permissive to cancer or sets up some early changes,” added Stephen Baylin, deputy director of the Sidney Kimmel Comprehensive Cancer Center at the Johns Hopkins School of Medicine.
The study, published in the Journal of Clinical Investigation, showed that in mouse embryonic stem cells, the methylation-inducing transgene construct hypermethylated the p16 promoter and decreased p16 transcription after stem-cell differentiation, since the transgene motifs are known to regulate methylation during development.
“It’s a very clever paper, I think the results are very clean and clear. The p16 gene is known to be methylated very early in the cancer process. In the test tube, my lab has shown directly that the methylation of this particular region silences the gene, so I think all the dots have been connected and this was just a final cherry on the top of the cake, so to speak, to show this causality,” said Dr. Jones.
The results from the Baylor team show direct evidence for a driving role of p16 epigenetic silencing by promoter hypermethylation in tumor formation and progression. This could allow the use of commercially available demethylating agents to reverse methylation, a possible mechanism for cancer treatments in the future.